Halogen containing polymers, especially those containing chlorine, are used widely and have great commercial significance. Polyvinyl chloride (PVC), in particular, is used in packaging, siding, pipe, and many extruded shapes. Such large scale and diverse use of the halogen-containing polymers depends upon the incorporation therein of good head and light stabilizers. PVC, for example, is known to have a tendency to degrade upon prolonged exposure to heat and light during processing and use. Darkening or other color change and the loss of tensile, flexural, and impact strengths are the results of such degradation. Unless good low-cost stabilizers are available for addition to the polymer composition, the service life of articles made from the composition is limited, and its use severely restricted, as will be the conditions for making it.
One particularly troublesome form of degradation occurs when the polymer composition is processed into articles by methods employing heat to melt or soften the polymer. A color change can occur during the first few minutes at high temperatures (e.g., from about 175.degree. to about 200.degree. C.) and it is commonly referred to as early color or early discoloration. The avoidance of such early color is notably important in the manufacture of plastic pipe and siding. It is, of course, also important to prevent or reduce discoloration and deterioration of the polymer during extended exposure to high temperatures, which can lead to sudden and catastrophic degradation into a pitch-like abrasive material and cause the formation of corrosive materials such as HCl inside the fabricating equipment. The inner, highly polished surfaces of the equipment can thus be rendered essentially useless. A particularly troublesome instance of such sudden degradation often occurs when zinc compounds are employed as stabilizers; the phenomenon is known as "zinc burn" in the PVC industry.
An example of zinc burn is illustrated in U.S. Pat. No. 4,515,916, wherein a PVC composition containing zinc bis-(octyl thioglycolate) as a stabilizer showed such burn after 3 minutes on a two roll mill at 193.degree. C.